Display screen or portion thereof with a graphical user interface



FIG. 1 is a front view of a display screen or portion thereof with a graphical user interface showing a first embodiment of my new design; and,

FIG. 2 is a front view of a display screen or portion thereof with a graphical user interface showing a second embodiment of my new design.

The outermost broken line rectangle illustrates a display screen or portion thereof and forms no part of the claimed design. The return arrow icon in FIGS. 1 and 2 forms no part of the claimed design. The remaining broken lines illustrate portions of the graphical user interface and form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 